From German Offenlegungsschrift No. 21 45 116, which corresponds to U.S. Pat. No. 3,820,643, there is known a printing head for combined alphanumerical characters and/or code symbols, wherein two printing bars with printing surfaces of different widths are provided for printing the bar code. The printing bars are mounted within a housing of the printing head in sliding engagement with one another and adapted to be actuated independently of one another by means of two reciprocating solenoids. Each printing bar serves for printing a bar of predetermined width, so that the printing head is suitable only for printing a bar code having bars of two different widths. This printing head is relatively bulky, as each printing bar has to be provided with a separate actuating solenoid, the strength of which has to be selected in accordance with the respective bar width. Another disadvantage results from the two printing bars being in sliding engagement with one another. Moreover the imprint of the larger-width printing bar tends to become blurred due to the lower pressure per unit of area.
A printing head known from German Offenlegungsschrift No. 24 15 214 also comprises two printing bars guided adjacent one another within a cylindric housing and adapted to be actuated by solenoids, with the two printing bars contacting one another adjacent their free ends. The free ends of the printing bars each carry a bar printing type of different width corresponding to two imprint widths of the bar code to be printed. In addition to their individual actuation, the printing bars may be simultaneously actuated for printing a bar the width of which corresponds to the sum of the imprint widths of the two printing bars. The wider printing bars tend to produce somewhat blurred imprints. Aside from the rather bulky construction with two actuating units and the corresponding return means, this embodiment suffers from the disadvantage that its employ is restricted to a bar code with predetermined bar widths numbering no more than three. A further disadvantage results from the frictional engagement between the two printing bars, at least adjacent their free ends, whereby the quality of the resulting imprints is further reduced.
From German Offenlegungsschrift No. 22 16 967 , which corresponds to U.S. Pat. No. 3,834,506, there is also known a printing head comprising two printing bars guided parallel to one another within a housing and provided with bar printing types at their forward ends. This embodiment suffers from the same disadvantages as explained above, in that its employ is also restricted to the printing of a bar code having bars of only three predetermined widths, and in that the printing bars are in frictional engagement with one another. This printing head is thus unsuitable for printing a bar code having bars of variable width or more than three different bar widths. The structural dimensions of a printing head employed for printing a bar code are of considerable importance, inasmuch as the printing head is usually employed in connection with a needle printing device for printing alphanumeric characters. In the case of labels or similar information carriers, it is desired that the printed information be arranged with the highest possible density, i.e. as closely above one another as possible, as seen transversely of the direction of movement of the information carrier. As the needle printing device requires a rather complicated mechanical structure and actuation of the printing needles should be substantially linear without any reversing points, the structure of the printing head should be as compact as possible, so that the mechanism of the needle printing assembly remains simple and effective, and so that the printed information can be arranged in the smallest possible area.
It is an object of the invention to provide a printing device of the type set forth in the introduction, which consists of the smallest possible number of components and is of lightweight and compact construction, while being adapted to be employed for printing bar codes of any type, i.e. bar codes having selectively variable bar widths and/or selectively any number of different bar widths, and which produces a particularly high quality of the printed bar code.